1. Field of the Invention
The present invention relates to the manufacture of galvanized perforated steel sheet product and, in particular, to a process for perforating, temper rolling and subsequently galvanizing a steel sheet.
2. Description of the Prior Art
In the metals producing industry, it is known that, if the surface of a steel product is left unprotected after finishing, such surface will be readily subject to oxidation or, as that process is more commonly known, rusting. In the oxidation process, the surface of a steel product is transformed from a unitary steel surface to one having visible areas of oxidation over the steel. The oxidation of a steel product serves to decrease its strength and to severely degrade its appearance, and eventually destroy the product.
One process which has been developed in an effort to prevent oxidation is galvanizing. In a galvanizing operation, a layer of sacrificial zinc material is applied over the steel's surface. An intermediate layer comprised of a zinc-steel alloy is formed at the zinc-steel interface. As such, the zinc covers the steel's surface and prevents the oxidation of the steel by allowing instead the oxidation of the zinc, which proceeds at a markedly slower pace than the oxidation of the steel.
While it is known to galvanize or otherwise protect the surfaces of various steel products prior to or following their formation into more usable forms, it has not been heretofore known to perforate a steel sheet or strip prior to galvanizing and eventually painting and forming into finished products. Applicants have discovered that a galvanized perforated steel sheet product may be effectively produced by sequentially temper rolling the steel sheet, perforating the sheet to a considerable extent across its surface, again temper rolling the sheet to reduce any burrs and shape defects produced in the sheet by the perforating process, galvanizing the perforated sheet, optionally temper rolling the sheet to smooth the surface and minimize the appearance of spangles in the surface of the sheet and optionally coating the sheet.
That the prior art does not contemplate the process discovered by Applicants is obvious from a review of such art. In an article by M. K. Hettleman entitled "Designing and Fabricating Steel Structures and Assemblies to be Hot Dip Galvanized After Fabrication", Materials Protection and Performance, Feb., 1972, pp. 37-40, the author states that bending, forming and punching of steel structures should be done before galvanizing. Id. at 40. However, such article does not disclose or suggest the perforating of a steel sheet across a substantial portion of its surface and then temper rolling the sheet prior to galvanizing.
That those skilled in the art were unaware of the benefits of perforating a steel sheet prior to galvanizing it is shown from a review of German Pat. No. 31 17 982. That patent, laid open in 1982, relates to a process for producing galvanized, perforated sheet steel which comprises, in sequence, galvanizing the sheet, perforating the sheet and then regalvanizing the sheet. As such, the German Patent shows that it was previously unknown in the art to first perforate the sheet and then galvanize it to avoid such redundant operations.
One patent in which a metal object was perforated prior to the application of a coating metal substance thereover is Great Britain Pat. No. 14,071. According to that patent, a metal object is perforated to provide large perforations, e.g., 1/4to 3 inches in diameter, and then a coating of lead is applied thereto. However, the British Patent clearly states that the perforations are filled with the coating lead so as to provide pins joining the coatings on the outer sides of the object. Hence, the British Patent does not teach perforating and subsequently galvanizing a steel sheet to provide a final galvanized, perforated steel sheet product.
As noted above, the subject invention is directed toward a method for producing a galvanized perforated steel sheet product in which the sheet is perforated, then temper rolled and then galvanized. Such process overcomes, among others, the above-discussed problems present with prior art processes and which efficiently and effectively produces a galvanized perforated steel sheet product. In particular, the present invention provides a perforated steel sheet having a protective metallic coating over its entire surface, whereas, in accordance with the prior art practice of galvanizing and then perforating a steel sheet, the perforating process removes the galvanized coating from the inner surfaces of the perforations.